(A) Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a light-emitting device having a concave-convex photonic crystal and a reflector.
(B) Description of the Related Art
FIG. 1 shows a conventional light-emitting device 10. The light-emitting device 10 includes a substrate 12, a light-emitting chip 14 positioned on the substrate 12, a fluorescent material (with a refractive index of 1.5-1.6) 16 positioned on the light-emitting chip 14, and a transparent cover (with a refractive index of 1.5) 18. Light beams 20 (e.g., an ultraviolet light) can excite the fluorescent material (e.g., RGB fluorescent material) 16 to emit excited light beams 22, i.e., red, green and blue light beams, which are mixed to form a white light 24. The light beams 20 produced by the light-emitting chip 14 must pass through the fluorescent material 16 and the transparent cover 18, and then can propagate outside the light-emitting device 10, for example, propagate to the air (with a refractive index of 1).
FIGS. 2(a) and 2(b) show the simulated output light intensity of a conventional light-emitting device 10. If the incident angle (θi) of the excited light beams 22 irradiated into the transparent cover 18 is larger than 45 degrees, the simulated output light intensity of the light-emitting device 10 is quite low (less than 0.05). As the excited light beams 22 are propagated from a high-refractive-index material to a low-refractive-index material, internal total reflection and high index waveguide losses occur. The internal total reflection causes the excited light beams 22 having an incident angle larger than the critical angle (about 45 degrees) to reflect completely back to the interior of the light-emitting device 10 by the transparent cover 18, and cannot be propagated outside the light-emitting device 10. In other words, most of the excited light beams 22 are reflected or absorbed inside the light-emitting device 10, which leads to serious losses such that the light output efficiency of the light-emitting device 10 is quite low.